1. Field of the Invention
The present invention relates to a nitrogen oxides (NOx) purification apparatus provided with a "lean" NOx catalyst which is defined as an NOx purification catalyst capable of purifying NOx under excess-oxygen conditions such as in exhaust gas from an internal combustion engine operated at lean air-fuel ratios.
2. Description of the Prior Art
To improve fuel economy and to suppress exhaust of carbon dioxide, thereby reducing global warming, engines capable of fuel combustion at lean air-fuel ratios (lean burn engines) are being developed and are in actual use today. Since a conventional catalyst (three-way catalyst) cannot reduce and purify NOx at lean air-fuel ratios, there is a need to develop a catalyst or system that can purify NOx even at lean air-fuel ratios.
Japanese Patent Publication HEI 1-139145 proposes a copper (Cu)/zeolite catalyst in which copper is exchanged on a zeolite carrier and which is capable of purifying NOx at lean air-fuel ratios in the presence of hydrocarbons (HC). To use such a lean NOx catalyst as an NOx purification apparatus for internal combustion engines, a system wherein the lean NOx catalyst can operate at a high NOx purification rate should be developed.
In this meaning, Japanese Patent Application HEI 2-317664 filed Nov. 26, 1990 proposes an exhaust gas purification system wherein two lean NOx catalysts are arranged in parallel with each other in an exhaust system of an internal combustion engine and exhaust gas flow is switched so that, when exhaust gas is flowing through one lean NOx catalyst, exhaust gas flow through the other lean NOx catalyst is stopped. When the exhaust gas flow is switched to flow through one of the two catalysts, the temperature of the one catalyst increases accompanied by a momentary increase in the NOx purification rate. By repeating switching of exhaust gas flow, the increase in the NOx purification rates is repeatedly produced so that the NOx purification rate of the system increases.
However, it has been found in further tests that almost no increase in the NOx purification rate of the system is seen just after the space velocity of exhaust gas at the lean NOx catalyst changes from a low velocity to a high velocity such as occurs just after an idling condition changes to an acceleration condition. This is true even though the exhaust gas is merely switched between the two lean NOx catalysts.
This suggests that an NOx purification rate (NOx conversion) of a lean NOx catalyst is affected not only by a change in the catalyst temperature but also by a space velocity of exhaust gas at the catalyst.